In gas turbine engines, hot gases from a combustor are used to drive a turbine. The gases are directed across turbine blades which are radially connected to a rotor. Such gases are relatively hot. The capacity of the engine is limited to a large extent by the ability of the turbine blade material to withstand the resulting thermal stress. In order to decrease blade temperature, thereby improving thermal capability, it is known to supply cooling air to hollow cavities within the blades. Typically one or more passages are formed within a blade with air supplied through an opening at the root of the blade and allowed to exit through cooling holes strategically located on the blade surface. Such an arrangement is effective to provide convective cooling inside the blade and film-type cooling on the surface of the blade. Many different cavity geometries have been employed to improve heat transfer to the cooling air inside the blade. For example, U.S. Pat. Nos. 3,628,885 and 4,353,679 show internal cooling arrangements.
One technique for improving heat transfer is to locate a number of protruding ribs along the interior cavity walls of the blade. By creating turbulence in the vicinity of the rib, heat transfer is thereby increased. In the past, such turbulence promoting ribs have been disposed at right angles to the cooling airflow. Such rib orientation is shown, for example, in U.S. Pat. No. 4,257,737. One problem with the use of turbulence promoting ribs perpendicular to the airflow is that dust in the cooling air tends to buildup behind the ribs. This buildup reduces heat transfer.
Turbulence promoting ribs also affect pressure and flow rate within the blade. It is imperative that the exit pressure of cooling air at the cooling holes exceed the pressure of the hot gases flowing over the blades. This difference in pressure is known as the backflow margin. If a positive backflow margin is not maintained, cooling air will not flow out of the blade, and the hot gases may enter the blade through the cooling holes thereby reducing blade life. Over and above the benefit of maintaining a positive backflow margin, a high exit pressure at the exit holes provides the benefit of imparting a relatively high velocity to the cooling air as it exits from these holes. Since most of these holes have a downstream vector component, a smaller energy loss from the mixing of the two airstreams or greater energy gain, depending on the magnitude of the air velocity, results thereby improving engine efficiency.
To ensure that exit pressure is sufficiently high, two criteria must be satisfied. First, pressure delivered to the cooling air inlet to the blade must be high. Second, the decrease of pressure between the inlet and exit must be low. This second criterion, known as pressure drop or delta p, is proportional to the friction factor inside the blade and the square of the flow rate. Delta p shows improvement as the friction factor decreases. The friction factor is affected in part by the geometry at the cooling passage walls. For instance, turbulence promoting ribs increase the friction factor by increasing shear stress which creates vortices behind the ribs.
Turbulence promoting ribs therefore simultaneously improve heat transfer while worsening pressure drop.